The ability of a pathogen to breach the highly polarized host mucosal epithelial barrier is an early and critical step in the pathogenesis of microbial infections. This event is particularly crucial for opportunistic pathogens such as Pseudomonas aeruginosa (PA), one of the most virulent opportunistic pathogens of man. Our long-term goals are to understand how pathogens in general, and PA in particular, overcome the host epithelial barrier to cause human disease. PA binds to the apical (AP) surface of polarized epithelial cells and triggers fundamental changes in the host cell that allow bacterial internalization. During this granting period, we have discovered that PA induces a remarkable local change in the morphology of the AP surface to form a membranous protrusion composed of basolateral (BL) constituents. This protrusion is the likely conduit for bacterial entry. This profound shift in the local identity of the AP membrane is the first known morphologic landmark of the effect of PA on epithelial cell monolayers. Based on our data, we hypothesize that PA can exploit small defects in epithelial polarity at the mucosal barrier to initiative a series of events that result in a positive feedback loop induces a remarkable morphologic change in which AP membrane is transformed into BL membrane. A protrusion, with characteristics of a nascent AJ, is formed that functions as a conduit for bacterial entry. By manipulating epithelial cell polarity, PA is able to enhance its bacterial binding and entry, possibly by increasing the presentation of host cell receptors or other factors that promote internalization. Our aims are to (1) Determine which key regulatory and structural components of TJ and AJ are recruited to the protrusion. (2) Determine the temporal and functional relationship between protrusion formation and bacterial entry. (3) Perform mechanistic studies to uncover how PA hijacks host cell vesicular trafficking to redirect these junctional and BL components to the AP surface. (4) Identify the bacterial determinants of protrusion formation. Together, these studies will comprehensively dissect the interactions between PA and host cell epithelium. They will identify host factors that the bacteria exploit to cause disease. The use of pathogens to study fundamental processes in mammalian biology, such as the establishment and the maintenance of epithelial polarity, has broad ramifications for many fields in addition to pathogenesis, including cancer and developmental biology. Furthermore, these host cell factors may serve as novel targets for the development of anti-bacterial therapeutics; because the drug targets host but not bacterial molecules, they are much less likely to engender resistance compared to conventional anti- microbial therapies.